There are many circumstances where it is desired to make measurements of the properties of pores in a porous material. One such is the testing of samples of porous rock.
It is conventional practice when drilling through underground rock to drill around a central cylinder of rock which is subsequently detached and brought to the surface as a sample, habitually referred to as a rock core. Rock cores may be subjected to various testing procedures at the surface, including tests to determine sizes of pores. For most natural porous media the pores have a somewhat complex geometry. Suggested representations of the geometry have included a ball-and-stick model and a series of intersecting spheres. Information on the size of pores present in oil-bearing rock is important for the understanding of fluid transport and hence oil recovery.
Mercury intrusion porosimetry (MIP) is a standard analytical method for measuring a distribution of pore sizes in a porous material. Mercury is routinely used for this test because it does not wet the surface of the sample. Liquid mercury is forced into a small sample of rock under elevated pressure which opposes capillary pressure resulting from ingress of non-wetting liquid into pores. The total porosity of the sample is determined as the total volume of mercury injected. A measurement related to pore size can also be made because the pressure required to force the non-wetting liquid mercury into an empty pore rises as the pore access size (the size of the confining space through which the mercury must pass to fill the void) diminishes. For most natural porous media this access size does not represent overall pore size but generally corresponds to the pore throat diameter. A size distribution is normally determined by increasing the pressure in steps and observing the cumulative volume intruded at each step. This procedure can produce a graph or histogram of pore volumes plotted against size.
MIP is normally carried out using a sample which has previously been placed under vacuum, so that the mercury is intruded into empty pores. However, it can also be used in other experimental procedures, for instance to determine the capillary pressures present in a sample as a non-wetting liquid displaces air. Intrusion porosimetry has also been carried out with other liquids as alternatives to mercury, sometimes seeking to use a liquid which does not have toxicity issues associated with it. However, mercury continues to be the preferred non-wetting liquid for much intrusion porosimetry.
A more recent method for determining pore size distributions is by measurements of nuclear magnetic resonance (NMR). Several NMR methods have been used. For example Davies et al in J. Appl. Phys. vol 67 pages 3163-3170 and pages 3171-3176 (1990) related the spin-lattice relaxation time T1 to distribution of pore size which was taken as size of an equivalent sphere. Mitchell et al in J. Phys.D: Appl. Phys. vol 38 pages 1950-58 (2005) state that the spin-spin relaxation time T2 of a liquid saturating a porous medium will be inversely proportional to the surface-to-volume ratio of the pores. However, this use of relaxation time is hindered by a lack of knowledge regarding the relaxivity, which affects the scale of the results. Relaxivity is dependent on properties of the rock sample including the density of paramagnetic species in the rock matrix (present, in particular, in clays) and the diffusion of the liquid molecules to and on the solid surface. Relaxivity has been seen to vary considerably even between chemically similar materials.